Examples of hydraulic actuating devices for a convertible top assembly of a motor vehicle are described, inter alia, in EP 0 693 389 and DE 38 26 788. In these known actuating devices, the feedback means which feed back information relating to the sequence of movements of the hydraulic actuators are designed as position sensors arranged at the actuators in the form of switches which are actuated when the associated actuator reaches a defined position. The position sensors are limit-position sensors which detect whether the associated hydraulic actuator has reached one of its limit positions. In the known devices, the output signals from these position sensors are used in particular as feedback signals in the control circuit for controlling the sequence of movements of the various parts of the convertible top assembly, such as the convertible top itself, which may comprise a plurality of separately actuable components, and furthermore possibly a tonneau cover, a boot lid, latches, windows or, for example, a roll bar of the vehicle.
Limit-position sensors are reliable and relatively inexpensive. However, in practice a movable component of a convertible top assembly often does not have an accurately defined limit position, but rather a range within which the limit position may vary as a function of, for example, manufacturing tolerances and play in the structure, (thermal) deformation of the components of the convertible top assembly or the compressibility of seals between the convertible top and the body or between parts of the convertible top. For this reason, with actuating devices of this type it is currently customary to arrange a limit-position sensor in such a manner that the sensor is activated a short distance from the limit position which is actually intended, for example 10 millimeters before this limit position, as seen in the direction of movement of the actuator. This limit-position sensor remains activated during the further displacement of the actuator towards the said limit position. Then, as soon as the limit-position sensor indicates to the control unit that the actuator has activated the limit-position sensor, the control unit causes that the said actuator remains hydraulically energized for a predefined time. This time lag is selected in such a manner that the actual limit position is reached under all circumstances. In practice, this may mean that a time lag of, for example, 0.5 second is adopted. However, depending on conditions, this time lag may be excessively long. By way of example, the actual limit position may be reached after 0.2 second. This in fact means waiting for an unnecessary 0.3 second. On the market, it is desirable for a sequence of movements of a convertible top assembly to be carried out as quickly as possible and with a smooth movement which is not jerky, time gains of one or more seconds being relevant to the complete opening or closure of the convertible top. Particularly if one component, for example the convertible top, of the convertible top assembly is only actuated when the movement of another component, for example the tonneau cover, has been completed, the abovementioned situation causes an undesirable delay and means that the movement of the convertible top assembly is not smooth.
In a hydraulic actuating device for a convertible top assembly of a motor vehicle, it is also known, for example from EP 0 550 952, to use one or more position sensors which are able to detect the position of the hydraulic actuator or the associated movable component of the convertible top assembly over a defined range. Position sensors of this type are considerable more expensive than the limit-position sensors described above and, moreover, do not offer a practical solution to the abovementioned problem. By way of example, the question of whether or not a convertible top of a motor vehicle has completely closed is a matter of millimeters at the location where the convertible top adjoins the body, even though a compressible seal does offer some compensation. If, in this case, the position sensor is, for example, an angle sensor which is arranged between two pivoting components of the convertible top structure, it would have to be an accurate angle sensor in order to detect the relevant distance with sufficient accuracy. In order nevertheless to be able to use less accurate sensors, in practice it is chosen to detect a limit position by analysing whether the feedback signal from the angle sensor used remains constant for a defined time, so that it is certain that the actuator and/or the associated part of the convertible top assembly has reached its limit position.
Another known embodiment of the feedback means in a control system for a hydraulically actuated convertible top assembly of a motor vehicle is described in U.S. Pat. No. 5,724,878. In this case, the feedback is primarily based on measuring the actual pressure in the hydraulic system. This pressure measurement may possibly be used in combination with limit-position sensors, as described in U.S. Pat. No. 5,724,878. However, this requires the use of a high-quality pressure gauge, in view of the required accuracy, service life and operating conditions. A pressure gauge of this type is undesirably expensive. EP 0 916 535 proposes special actuators which can be used in combination with this pressure measurement.